5. SUMMARY

In Section 4, it was argued that the
sidedness, transverse brightness profile and field
structure properties of strong-flavor jets are best explained if they
have relativistic
bulk velocities. This leads to the following identifications for the
structures identified in Section 2:

Strong-flavour jets are highly-collimated, relativistic (and
therefore necessarily
supersonic) flows. They occur in the nuclei of most, if not all radio
galaxies and quasars.

Central components are the optically-thick bases of strong-flavor jets.

Weak-flavour jets are supersonic (or, at least, transonic),
sub-relativistic flows. They
are formed by the smooth deceleration of strong-flavor jets.

Hot-spots in FRII sources are the post-shock flows formed when a
highly-supersonic
(and, therefore, nearly always strong-flavor) jet terminates by
collision with the
surrounding medium. The flow speed of material emerging from a hot-spot may be
relativistic in some cases, particularly in the most powerful
sources. Hot-spots in
FRI sources such as wide-angle tails may be formed by a different
mechanism, but also occur where a strong-flavor jet disrupts.

Bridges are subsonic backflows from the end of a jet of either flavor.

They decelerate on kiloparsec scales in a way determined by jet
power and environment.

A plausible mechanism for deceleration in twin-jet sources is
entrainment of external material into a transonic flow
(Begelman 1982;
Scheuer 1982;
Phinney 1983;
Komissarov 1992).
Mass loss from stars may also provide enough material. The jet
can only decelerate in this way provided that it can be recollimated by
the external
pressure gradient. This sets an upper limit to the kinetic luminosity of
the jet of 1035-36 W for plausible galactic parameters
(Phinney 1983;
compare the estimates given earlier).

Strong-flavor jet bases have
j 1 and are therefore
one-sided. They are
limb-brightened and have longitudinal apparent magnetic field because we
see the boundary layer of a decelerating flow.

Weak-flavor jets are transonic flows with
j <<
1. They may well be turbulent, as suggested by Bicknell.

Tails and bridges are subsonic. Tails are likely to be formed if
the jet entrains
enough material to bring the density contrast close to 1 or if buoyancy
causes the
central region of a bridge to be pushed outwards.

Jets in FRII sources (with the possible exception of the weakest
examples) have
j 1 on all scales.

They have << 1
and are highly supersonic.

The post-shock flow is mildly relativistic in high-power sources
(Laing 1989) but
probably not in low-power ones
(Black et al. 1992).
This may require the shock
to be oblique, consistent with the recessed appearance of many bright, compact
hot-spots.

The very simple picture presented here is conventional in many
respects. The essential
new point is the suggestion that all strong-flavor jets and jet
bases are relativistic
and that the marked differences between the strong and weak flavors
result directly
from the importance or otherwise of beaming and aberration effects. The
initial speeds
of all kiloparsec-scale jets are then very similar and the differences
in radio luminosity
and morphology depend on the jet's power and the environment through which it
propagates. In particular, the division between FRI and FRII sources may
be determined by the ability of a jet to decelerate smoothly without
disruption.

Acknowledgements

I thank the ST ScI for travel expenses, Geoff Bicknell, Paola Parma and
Alan Bridle
for their thoughts and the editors for their extreme patience.